Field of the Invention
The present invention relates to an objective lens and transmission electron microscope.
Description of Related Art
In transmission electron microscopy, a magnetic objective lens is known as an objective lens for focusing and imaging an electron beam. Magnetic objective lenses have been so improved that the focal distances of the lenses are reduced by generating a strong magnetic field in or near a sample.
However, where a sample that is sensitive to a magnetic field such as a magnetic sample is observed with a transmission electron microscope, there is the problem that the magnetic field produced by the objective lens varies the magnetic characteristics of the sample. If the magnetic field of the objective lens is placed at a distance remote from the sample in order to protect the sample from the effects of the magnetic field, the focal distance of the objective lens is increased. This will degrade the resolution of the electron microscope. Accordingly, it has been difficult to observe a magnetic sample appropriately at high resolution using an objective lens of the electromagnetic type.
Furthermore, a Lorentz force (Maxwell stress) exerted by the magnetic field of an objective lens acts on a magnetic sample in such a way as to pull the sample toward the objective lens. This makes it difficult to hold the magnetic sample at a given position of observation.
As regards this problem, the objective lens set forth in JP-A-2005-32588 is so designed that magnetic fields produced respectively by first and second electromagnetic lenses cancel each other out to zero in the region where a sample is placed. More specifically, the magnetic fields are produced symmetrically with respect to the sample placement surface by the first and second electromagnetic lenses such that the magnetic field component at the position of the origin of the sample which is in the vertical direction (i.e., in a direction along the optical axis) is prevalent. In addition, the vertical components of the magnetic fields respectively produced from the two lenses cancel out each other at the position of the origin of the sample. Consequently, the magnetic field at the position of the origin of the sample is made substantially zero in strength.
In this objective lens, the magnetic field strength in the region where the sample is placed can be substantially made zero and, therefore, a magnetic sample can be observed without varying its magnetic characteristics. Furthermore, in this objective lens, it is possible to prevent the magnetic sample from being drawn toward the objective lens by a Lorentz force.
In the objective lens disclosed in JP-A-2005-32588, the magnetic field strength at the position of the origin of the sample where the optical axis and the sample placement surface intersect each other can be made substantially zero. However, at positions lying off the position of the origin of the sample, the horizontal components of the magnetic fields (which are perpendicular to the optical axis) produced by the first and second electromagnetic lenses do not cancel out each other but rather combine additively at some locations. Consequently, in the objective lens disclosed in JP-A-2005-32588, the sample may be affected by the effects of horizontal magnetic fields.